Scientific and clinical studies have relatively recently been performed with respect to diffuse optical tomography, which maps attenuation and scattering coefficients, which are then related to chemical processes which are known to alter such coefficients. Compared to x-ray tomography, which maps only the affective atomic density that attenuates x-ray transmission, diffuse optical tomography can be employed to map chemical structure when it is applied in spectral regions that are absorbed by chemical bonds.
Conventionally, diffuse optical tomography has been utilized in mammography systems. A conventional diffuse optical tomography for mammography system includes an annular fiber-optic probe array that comprises a plurality of optical fibers. In such a system, one optical fiber is placed in contact with a breast and caused to transmit an optical signal having a wavelength near the absorption resonance of hemoglobin (750-900 nm), while the other optical fibers in the fiber-optic probe array, also in contact with the breast, collect light exiting the breast and pass such light to photodetectors, which then produce a photocurrent. A lock-in amplifier is coupled to a single photodetector (associated with a single optical fiber from the “sensing” optical fibers), and measures the phase and amplitude of the photocurrent produced by the photo-detectors. Typically, only one lock-in amplifier is used in a conventional diffuse optical tomography for mammography system, as lock-in amplifiers tend to be relatively large in size and relatively expensive.
Since such a mammography system includes a single lock-in amplifier, photocurrent from the photo-detectors coupled to the sensing optical fibers must be read serially. Once the photocurrent from all of the photodetectors respectively associated with the sensing optical fibers have been read, one of the sensing optical fibers is caused to be the “illuminating” optical fiber to provide illumination, while the previous illuminating optical fiber becomes one of the sensing optical fibers. This process is repeated until at least half of the optical fibers have been selected to be the illuminating optical fiber. The set of photocurrent phase and amplitude measurements composes a tomogram at a particular wavelength at the particular plane through the breast at which the probe array is located.
Subsequently, the wavelength of illumination output by the optical fibers is altered to be within the hemoglobin absorption resonance, and the process described above is repeated such that a new tomogram is generated at the same plane of the breast. Typically, the transmission wavelength will be stepped through several values within and near the absorption resonance, thus developing a set of tomograms corresponding to the differing absorption properties of the breast tissue. By applying tomography inversion techniques, the measured phase and amplitude of the photocurrent can be processed to indicate the location of absorbers within the measurement plane. By sliding the annular fiber-optic probe array to a new position along the tissue, a complete set of tomograms can be obtained, finally yielding a three-dimensional plot of absorber density.
It can be ascertained that several deficiencies are associated with such a conventional mammography system. Specifically, computing a complete set of tomograms can take several minutes, as the annular fiber-optic array must be placed at multiple positions and the role of illuminating fiber-optic must be altered. During this time, natural patient motion such as respiration may disturb the registration of the fibers to the breast, resulting in blurred or erroneous tomographic reconstruction. Additionally, the annular fiber-optic array must be in contact with human tissue, which often can cause the patient discomfort and may bring forth hygiene concerns.